1. Field of Invention
This invention generally relates to the production of Radio Frequency Identification Device (RFID) tags, specifically to the reliable testing of finished RFID tags and labels at high speed in a production environment.
2. Prior Art
RFID tags and labels have a combination of antennas and analog and/or digital electronics, which may include for example communications electronics, data memory, and control logic. RFID tags and labels are widely used to associate an object with an identification code. For example, RFID tags are used in conjunction with security-locks in cars, for access control to buildings, and for tracking inventory and parcels. Some examples of RFID tags and labels appear in U.S. Pat. Nos. 6,850,080 (Hiroki 2005), 6,429,831 (Babb 2002), 6,940,408 (Ferguson et al. 2005), 6,784,789 (Eroglu et al. 2004), 6,535,175 (Brady et al. 2003), 6,827,817 (Bleckmann et al. 2004), 6,780,265 (Bleckmann et al. 2004), and 6,451,154 (Grabau et al. 2002).
RFID tags are wireless devices which communicate only with wireless base stations. Those base stations are commonly called RFID readers.
Radio Frequency Identification Device (RFID) tags are manufactured through the execution of a number of steps:
(a) Integrated Circuit (IC) chips are manufactured and tested. The testing is performed according to many methods, including those disclosed in U.S. Pat. Nos. 6,850,080 (Hiroki 2005) and 6,429,831 (Babb 2002).
(b) The IC chips are attached to antennas. The resulting assemblies, commonly called “inlays,” are tested. Testing is performed according to many methods, including those disclosed in U.S. Pat. Nos. 6,940,408 (Ferguson et al. 2005), 6,784,789 (Eroglu et al. 2004) and 6,535,175 (Brady et al. 2003). Most tests on inlays are performed with specialized, expensive, equipment that measures the electrical characteristics of the inlay rather than performing a direct functional test of the finished inlay. Tested Inlays are commonly delivered as a continuous web, on a roll, for further processing.
(c) Inlays are cut from the roll and inserted into paper labels, cloth labels, plastic housings and other assemblies needed for various applications, through a process generally called “conversion.” The resulting assemblies, commonly called “tags,” are tested. Testing is performed according to many methods, including a number of slow, labor intensive methods described below. Finished RFID tags are commonly delivered as a continuous web, on a roll, for further processing. U.S. Pat. Nos. 6,827,817 (Bleckmann et al. 2004) and 6,780,265 (Bleckmann et al. 2004) disclose methods for manufacturing RFID tags but do not discuss testing. U.S. Pat. No. 6,451,154 (Grabau et al. 2002), “RFID Manufacturing Concepts,” states minimally, “The paper web moves past a station where conventional RFID read/writer equipment is provided. The equipment practices at least one of verifying the functionality of, or programming, the chips of inlets prior to the formation of a composite web,” this being the only mention of testing in the patent. Such functional testing is not trivial or obvious to implement. This patent does not disclose enough information to develop an effective subsystem for RFID tag testing.
Common Practices
Many enterprises pass finished RFID tags in front of an RFID reader and employ technicians to watch lights or other indicators that show that each tag is being read. Each tag that fails to read is then immediately replaced by the technician, or marked for later replacement. These manual test methods, while very slow and labor intensive, are widely used because they incur very low equipment cost on the enterprise.
Several vendors of RFID equipment have included in their equipment the ability to perform actions based on logical triggers. Some of these vendors have attempted to create RFID test systems by connecting an optical gap sensor to the trigger input of such an RFID reader. The theory of operation of such a system is that the gap between labels can drive the timing of any testing that needs to be done. However, those systems have a number of weaknesses. Most of those weaknesses involve timing and electrical requirements associated with the support of sensors and actuators. For example, the trigger inputs of most RFID readers are not fast enough to react to the signal produced by an optical gap sensor when the interlabel gap moves past the sensor at the high speeds common to production environments. One very important weakness of such a setup is the spatial relationship between the interlabel gap and the RFID tag within the label. An RFID test system based solely on optical gap sensing with no position detection will require constant mechanical adjustment of the optical gap sensor to accommodate physical differences between production runs. Further, such systems do not control the timing of their outputs. The RFID reader replies when its programmed actions are complete, with no regard for the timing requirements of the system to which it is connected. Some have proposed extending this design philosophy to the marking of bad RFID tags, simply by connecting a marker to the “bad tag” signal output of the RFID reader. For all the reasons above, this will not work fast enough or consistently enough for production environments.
Many enterprises purchase and use a special tester for RFID tags. These testers commonly incorporate mechanisms for positioning individual RFID tags on a continuous web in front of an RFID reader. The reader, controlled by a separate computer, then performs a series of tests on each RFID tag to be so positioned. At the end of the test, the web is advanced to the next tag position. Each tag that fails the test is marked for later replacement. This special purpose RFID test device has a number of disadvantages:
(a) Since the continuous web of RFID tags must be stopped in order to test each tag, the system wastes a lot of time stopping and restarting the continuous web.
(b) Since the tests are controlled by a separate computer, the system wastes a lot of time sending the same commands to the RFID reader for each tag tested.
(c) Since the tester is a separate device that stands alone from other devices, workers must transfer rolls of RFID tags to and from this device.
Antennas and Shielding
One challenge faced by designers of short range RFID systems, including test systems, printers, and label applicators, is antenna design. Most short range RFID systems use a small antenna similar in size to the RFID tag itself. These antennas must generally be positioned very close to the RFID tag in order to function correctly. That means that each tag must stop in front the antenna for the duration of the test, which further implies that no device based on these small antennas can be operated with continuous web motion.
Another challenge faced by designers of short range RFID systems, including test systems, printers, and label applicators, is antenna shielding. If one and only one RFID tag is to be tested, all other tags in the vicinity must be shielded from the reader, or the reader must be shielded from them. Most designs use reflective metal surfaces, absorptive ferrite materials, or a combination of reflective and absorptive elements. The approach has a number of drawbacks, including excess RF reflections, complexity and cost.